An electric storage device is an extremely important electric device that supports today's ubiquitous society due to its feature of taking out electric energy at any place and time when necessary. On the other hand, with the spread of portable devices such as video cameras, personal computers, cellular phones, potable music players and portable game devices, high capacities and miniaturization for electric storage devices (particularly, secondary cells) have been strongly required year after year. Of these, a lithium ion cell is high in energy density and high in power density per volume and per mass as compared with other electric storage devices, so its demands are increasing greatly as am electric storage devices satisfying the above-described needs.
Furthermore, recent global warming, atmospheric pollution, the exhaustion of oils, CO2 emission regulations, etc. raise issues, so an environmental load of automobiles is becoming a large problem. Thus, electric vehicles (EV), hybrid electric vehicles (HEV), fuel cell vehicles (FCV), etc. have been enthusiastically studied for development and practical application that can be one of the solutions for environmental measures (improvement of cleanliness), energy saving measures (improvement of fuel consumption), the next generation fuel measures (new energy development), etc. Attention has been paid, for example, to lithium ion cells, electric double-layer capacitors, and the like as their main power sources or auxiliary power sources, and their speedy applications have been studied.
Here, lithium ion cells generally have shapes such as cylindrical, angular, coin-like and laminate shapes. The insides of these cells have a construction in which an anode, a cathode, and a separator that is placed so as to separate the electrodes are spirally wound (wound type or spiral type), a construction in which alternate sheets of them are laminated or its similar construction (laminate type, stack type).
With an increasing demand for the above described electric storage devices and further needs for performance improvement, properties required for a separator making up an electric storage device are becoming a higher level.
Here, properties needed for a separator for the above lithium ion cell, as also described in Patent Document 13 below, primarily include separation properties, cell producibility, cell properties and the like.
Separation property is the most basic property needed for separators which requires that an anode is electrically separated from a cathode without short circuit and also the separator has ion permeability in a state impregnated with an electrolyte solution, further that the separator is inactive in electrochemical reaction environments (chemical resistance, oxidation and reduction resistances), and the like. In particular, for the prevention of the short circuit of an anode and a cathode, a separator is important to have no pin holes and no cracks.
Next, cell producibility is needed in particular when it is applied to wound type cells. In a step of winding a cell, electrodes are laminated to a separator to be made to wind them in a spiral form at a high speed. At this time, although the electrodes have concaves and convexes and may generate peeled materials during high speed winding, the separator wound at the high speed is required for not being broken due to the above concaves and convexes or the peeled materials, thereby not generating insulation failure of the cell. In other words, a high puncture strength is important for a separator. In addition, even in the cases other than a winding type, when the strength in the longitudinal direction (=lengthwise direction, flow direction, MD) is weak when spreading and winding of a separator including other steps of producing a cell, the film is elongated, wrinkled or broken in some cases (a person skilled in the art, when the phenomena are seen, regards the separator as being inferior in step passability, secondary processability or handling properties). Accordingly, the strength in a longitudinal direction also needs to be high. Like this, separators are important to be excellent in mechanical physical properties.
Cell properties require that current properties represented by charge and discharge performances at a large current (rate properties), charge and discharge performances at low temperature, and the like are excellent, that repeating of charge and discharge over a long period of time is possible (cycle properties), that a cell capacity can be maintained at high temperature (heat resistance), further that thermal runaway along with a rise in cell temperature due to overcharge or the like can be prevented (current shielding) (shutdown mechanism), and the like. Low internal resistance of a cell is important for improvement of rate properties, and when a same electrolyte solution is used, the thinner the separator, the higher the porosity, the larger the pore size, or the smaller the flexibility of the pore structure, the smaller the resistance tends to be. The selection of active materials for an anode and a cathode, and the construction inside a cell for improvement of loading density and the like are also important for cycle properties and heat resistance, and rare clogging of decomposed materials of an electrolyte solution in the surface opening of a separator, maintenance of an electrolyte solution poured into a separator, heat resistance of a separator itself, and the like are also important for cycle properties and heat resistance. The shutdown mechanism is a safety device of a cell, and importantly instantaneously melts and pore-encloses a separator at the time of temperature rise along with a runaway reaction to completely shutdown current and also importantly forms a continuous layer without breaking the separator to as high a temperature as possible after pore enclosure to continuously shutdown the current.
Because of these demand properties, presently, the separator for lithium ion cells primarily uses a chemically stable polyolefin microporous film represented by polyethylene or polypropylene.
The methods of forming pores of a microporous polyolefin film are generally roughly classified into a wet method and a dry method. The wet methods include an extraction technique that involves adding an extract to be extracted to polyolefin and finely dispersing and making a sheet and then extracting the extract to be extracted with a solvent or the like to form pores and, as required, having a step of carrying out stretching processing before and/after extraction, and the like (e.g., see Patent Document 1). The dry methods include a lamellae-stretching technique that involves carrying out low-temperature extrusion at the time of making a sheet by melt extrusion, and producing a non-stretched sheet having formed therein a special crystalline lamellae structure by taking high-draft, special melt crystallization conditions, and then primarily uniaxially stretching it to separate the stacked lamellae to form pores (e.g., see Patent Document 2, Non-patent Document 1). In addition, other dry methods include an inorganic particle technique that involves stretching a non-stretched sheet produced by addition in quantities of non-miscible particles such as inorganic particles to polyolefin to peel different material interfaces and form pores (e.g., see Patent Document 3). The others include a β-crystal technique that involves forming a low crystal density β crystal (crystal density: 0.922 g/cm3) during the production of a non-stretched sheet by melt extrusion of polypropylene, and stretching it to transit the crystal to a high crystal density α crystal (crystal density: 0.936 g/cm3) to form pores by means of the crystal density difference of the both (e.g., see Patent Documents 4 to 9, Non-patent Document 2).
The above β-crystal technique forms a large numbers of pores in a film after stretching, so a large amount of β crystal needs to be selectively formed in a non-stretched sheet prior to stretching. Because of this, the β-crystal technique uses a β crystal nucleating agent and importantly generates β crystal under specific melt crystallization conditions. Recently, a material having still higher β crystal formation capacity (e.g., see Patent Documents 10 and 11) is proposed as a β crystal nucleating agent as compared with a quinacridone compound used so far (e.g., see Non-patent Document 3), and a variety of microporous polypropylene films are proposed.
In addition, a method of producing a resin composition, a film or a pore-bearing film that contains 0.01 to 10 weight % of ultra high molecular weight polyethylene or polytetrafluoroethylene, has a β crystal content (K value) of 0.5 or more using an X ray and has a melt strength (MS) of 5 cN or more measured at 230° C., and other methods are also proposed (see Patent Document 12) for the purpose of improvement of low-temperature film producibility and thickness irregularity of a microporous polypropylene film produced by means of the β-crystal technique.
Further, many separators using a microporous polyethylene films are proposed that include, in addition to the above, for example, a polyolefin microporous film in which its average pore size and an average pore size of at least one surface thereof are in specific ranges and a separator for lithium ion cells constituted by it (see Patent Document 13), a polyolefin microporous film having a compression distortion factor and a puncture strength in specific ranges and a separator for lithium ion cells constituted by it (see Patent Document 14), a porous film including a polyolefin resin and having a pore structure parameter and a tensile strength in specific ranges and a separator for lithium ion cells constituted by it (see Patent Document 15), a cell separator containing a polypropylene microporous film which is produced from a precursor containing a beta nucleus and which has an electric resistance and a fracture strength in specific ranges (e.g., see Patent Document 16), and the like.    Patent Document 1: Japanese Patent No. 1299979 (claim 1)    Patent Document 2: Japanese Patent No. 1046436 (claim 1)    Patent Document 3: Japanese Patent No. 1638935 (claims 1 to 7)    Patent Document 4: Japanese Patent No. 2509030 (claims 1 to 8)    Patent Document 5: Japanese Patent No. 3443934 (claims 1 to 5)    Patent Document 6: Japanese Patent Laid-Open No. 7-118429 (claims 1 to 3, Examples 1 to 9)    Patent Document 7: Japanese Patent No. 3523404 (claim 1)    Patent Document 8: International Publication No. 02/66233    Patent Document 9: Japanese Patent Laid-Open No. 2005-171230 (claims 1 to 18, Examples 1 to 8)    Patent Document 10: Japanese Patent No. 2055797 (claims 1 to 8)    Patent Document 11: Japanese Patent No. 3243835 (claim 1)    Patent Document 12: U.S. Pat. No. 6,596,814 (claims 1 to 31, p. 2 paragraph 1, lines 18 to 50, Examples 1 to 3, Comparative Example 4)    Patent Document 13: Japanese Patent Laid-Open No. 2000-212323 (claims 1 to 3, Prior Art)    Patent Document 14: Japanese Patent Laid-Open No. 2000-212322 (claims 1 to 3)    Patent Document 15: Japanese Patent Laid-Open No. 2001-2826 (claims 1 to 8, Prior Art)    Patent Document 16: Japanese Patent Laid-Open No. 2000-30683 (claims 1 to 12, Examples 1 to 10)    Non-patent Document 1: Adachi et al., “Chemical Industry,” Volume. 47, 1997, pp. 47-52,    Non-patent Document 2: M. Xu et al., “Polymers for Advanced Technologies”, Volume 7, 1996, pp. 743-748    Non-Patent Document 3: Fujiyama, “Polymer Applications,” Volume. 38, 1989, pp. 35-41